Title: Redundant Roles of PRDM Family Members in Zebrafish Craniofacial Development.
Authors: Hai-Lei Ding, David E. Clouthier, and Kristin B. Artinger
Accepted: Developmental Dynamics; 14 October 2012
PRDM superfamily zinc-finger transcription factors play a critical role in Craniofacial development in Danio rerio (zebrafish).
Published in the Journal of Developmental Dyanamics, the Artinger lab has identified key genes that play a redundant role in orchestrating craniofacial development where a species of bony fish, the zebrafish Danio rerio, is used as their model. Because many birth defects involve defects in craniofacial cartilage development, the lab wants to determine the potential genes involved. Previous research in the lab has shown the importance of PRDM1a in zebrafish craniofacial development (Birkholz et. al., 2009). Prdm1a mutant embryos showed reduced neural crest cells needed for craniofacial development. This paper demonstrates that other genes in the PRDM superfamily (prdm 3, 5, and 16) play a redundant role in the development of the viscerocranium and neurocranium similar to prdm1a mutant animals previously described. Double mutant animals of prdm1a and prdm5 also revealed a more severe phenotype.
The viscerocranium is comprised of cartilaginous tissue, inclucing Meckel’s cartilage, contribute to the jaw and gills of the zebrafish. The neurocranium is comprised of cartilage that supports the brain and auditory capsule and is similar to the mammalian palate (Kimmel and Eberhart, 2008; Wada et. al., 2005). This study investigates how zinc-finger transcription factors 1a, 3, 5, and 16 affect craniofacial development. The researchers demonstrated the redundant function of these genes by using mutant animals completely deficient of specific prdm proteins as well as knocking down the expression of the gene products in interest using antisense oligonucleotides, or morpholinos (MOs).
Results: Five days post fertilization, zebrafish larvae were stained for cartilage and mounted. Controls show normal Neurocranial and viscerocranial development; however, morphant prdm3 and prdm 16 animals had significantly diminished structures both in the viscerocranium and neurocranium. Surprisingly, 20-30% of prdm3 morphants also demonstrated clefting of the ethmoid plate or palate. Moreover, prdm3 and prdm16 double morphant larvae resulted in a slightly more severe phenotype. Morphant larvae had reduced palatoquadrates including the pterygoid process, hyposymplectic, and shortening of mekels cartilage. Shotended trabeculae, ceratohyals (ch). Prdm3 MO animals also demonstrates a clefting of the anterior ethmoid in 20-30% of morphant larvae. (figure 2).
Prdm1a and 5 mutant animals were also tested to see their developmental phenotype. Prdm5 mutant animals were identified in a viral insertion screen and is predicted to be a full null. Both mutant larvae show diminished size and level of definition. Prdm 1 and 5 heterzygotes were crossed and prdm1/5 double mutants were isolated and observed. Double mutant animals had a profound phenotype demonstrating severe craniofacial anomalies (figure 5).
Conclusion: the data reveals that prdm3, 5 and 16 are needed for cranialfacial development in zebrafish. Knocking down or knocking out these zinc-finger transcription factors, lead to developmental malformations.
Questions: Why have redundant gene functions? Having redundant genes functions may be a way for important developmental pathways can assure the survival of the organism even when some genes lose their function. It also shows you how important this part of development is! This also complicates things for scientists since the phenotype does not always reveal the genotype definitively. Also, more severe phenotypes probably can mean more combinations of mutations rather than one single mutation.
Why zebrafish? Zebrafish are bony fish with high evolutionarily conserved gene sequence homology to humans. Gene homology allows scientists to use model animals to study developmental pathways and disease processes. Because the zebrafish lay large numbers of eggs external to the mother, this model can be a rapid and efficient way to screen for gene function. Overall, it is the hope that these models can help improve the treatment of detrimental craniofacial diseases in humans.